Lumbar Support Can Be Harmful To Your Health!
Proper Weight Bearing on the Seat Surface
by Dennis Zacharkow, PT
Two areas of the body that have evolved to withstand great external pressures are the heels of the feet and the ischial tuberosities (sitting bones). The ischial tuberosities are the most important areas for weight bearing on the seat, as they provide the important reference points for coordinated movement of the trunk and limbs (Fleischer et al., 1987; Forssberg and Hirschfeld, 1994). They are the pivot points for shifting the trunk forwards and backwards. A secondary region for weight bearing on the seat is the upper half of the back of the thighs (Bennett, 1928).
Figure 1: Areas of the body not suitable for weight bearing on the seat are the coccyx (1), the greater trochanters of the femurs (2), and the back of the lower thighs (3). The two major weight bearing areas should be the ischial tuberosities (4) and the upper half of the back of the thighs (5). Adapted from Bennett, H.E.: School Posture and Seating. Boston, Ginn and Company, 1928.
Three areas not suitable for weight bearing on the seat are (Figure 1):
- The coccyx (tail bone). Weight bearing on the coccyx is primarily due to excessive backward tilting of the pelvis. Secondary factors are sitting on a very thick, soft cushion or sitting with an excessive recline to the backrest.
The key to preventing the coccyx from weight bearing on the seat is sacral support, thereby preventing excessive backward tilting of the pelvis. As a result of sacral support, the weight bearing on the seat will be shifted forward to its proper location -- over the ischial tuberosities and the upper half of the posterior thighs (Zacharkow, 1988).
- The greater trochanters of the femurs (the bony projections adjacent to the hip joints). Based on their structure and function, the trochanters are completely unsuited for supporting the body weight in a sitting position (Helbig, 1978). Try sitting for an extended period of time in a car with an extreme bucket seat, or better yet on a toilet seat! (See Dennis Zacharkow's article "Bucket Seats: Why They Cause Pain and Discomfort.")
- The back of the lower thighs, just behind the knees. Pressure in this region of the thighs obstructs venous blood flow from the lower legs, resulting in lower leg swelling and discomfort.
- Bennett, H.E.: School Posture and Seating. Boston, Ginn and Company, 1928.
- Fleischer, A.G., Rademacher, U. and Windberg, H.J.: Individual characteristics of sitting behaviour. Ergonomics, 30: 703-709, 1987.
- Forssberg, H., and Hirschfeld, H.: Postural adjustments in sitting humans following external perturbations: muscle activity and kinematics. Experimental Brain Research, 97: 515-527, 1994.
- Helbig, K.: Sitzdruckverteilung beim ungepolsterten sitz. Anthropologischer Anzieger, 36: 194-202, 1978.
- Zacharkow, D.: Posture: Sitting, Standing, Chair Design and Exercise. Springfield, Thomas, 1988.
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